This invention relates to a position control or positioning control system responsive in general to a sequence of position commands supplied from an external source for intermittently controllably positioning a movable element at successively commanded positions. More specifically, this invention ralates to a position control system for either a carriage or a type or print wheel, drum, or cylinder of a serial printer of the type known as an impact type in the art.
An impact type serial printer for use in combination with an electronic digital computer comprises a carriage, a carriage motor for reciprocating the carriage along a predetermined path, and a position control system for controllably driving the motor so as to successively intermittently place the carriage at desired linear positions commanded by the computer. A rotary type wheel carrying a plurality of type elements is mounted on the the carriage together with a type wheel motor for rotating the type wheel. A similar position control system contrallably drives the type wheel motor to intermittently place the type wheel at desired rotational positions commanded also by the computer. Successively selected type elements are thereby placed at a printing position determined relative to the carriage. A position control system for controllably positioning a movable machine element, such as the carriage or the type wheel, to a commanded position comprises an increment encoder or position transducer mechanically or otherwise coupled to the motor. It is already known to make the encoder produce a pair of phase staggered or displaced incremental position signals, which is in effect representative of an angle of rotation of the motor and a current or instantaneous angular velocity thereof and hence a current position and an instantaneous speed of the movable element. The position signals are subtracted from the command signal so as to provide an error signal representative of a position error between the current and the commanded positions. The motor is controlled to render the position error zero. It is to be noted here that the commanded positions are selected from a plurality of predetermined positions with repetition allowed.
In U.S. Pat. No. 3,954,163 issued to Andrew Gabor on May 4, 1976, a position control system for an impact serial printer is disclosed wherein each of a motor for a carriage and another motor for a rotary print wheel is controllably driven by a first motor drive signal at first until the movable element reaches a point spaced a predetermined distance from each commanded position and then by a second motor drive signal until the element eventually rests at the commanded position. By the use of a velocity logic unit depicted in FIG. 7 of Gabor at 67 and in his FIG. 12 in two parts at 157 and 158, an analog velocity signal is produced from the position signal to indicate a current speed of the element, namely, a current angular velocity of the motor. An analog velocity reference signal is produced in response to an external control signal and to the position signal. The first motor drive signal is given by algebraically summing the velocity reference signal and the analog velocity signal. An analog position error signal is produced from an external command signal and the position signal to indicate a position error of the element. The second motor drive signal is given by algebraically summing the position error signal and the analog velocity signal. Use of analog signals, however, makes it difficult to achieve expected results in productivity and maintenance of the position control systems. The above-mentioned velocity logic unit is complicated in structure. Furthermore, it is rendered difficult to provide a compact system. This means that the position control system becomes bulky and accordingly highly expensive.